Page translated by Claude — switch to Italian to read the original article.
Sky-Watcher Star Adventurer: don’t call me a star tracker
I have been doing astrophotography for many years now, first with film, then with a DSLR, finally arriving at the monochrome CCD.
Like many astrophotographers I began my career, back in the days of chemical film, with shots on a fixed tripod, then moved on to piggyback photography with manual guiding and, before long, to prime-focus photography.
What I miss from that first period is precisely the wide-field images of celestial objects: these days I prefer to devote my sophisticated equipment to more “elaborate” images at medium to long focal lengths, rather than to a “plain” wide-field photograph of a constellation with a telephoto lens riding alongside the main instrument.
Over the years my equipment has gradually evolved and grown in quality and, above all, in quantity: tripod, mount, telescope, computer, extension cables, power tank, cameras and so on and so forth — and astrophotography sessions have become progressively more laborious and time-consuming.
Recently, therefore, I began to want a lightweight alternative for grab-and-go sessions: for example when you are on holiday with the family and you do not want, or cannot, bring the entire photographic kit with you, or simply to make the most of the little time available for photography. Hence the idea of buying a star tracker.
After weighing up various alternatives on the market, in June 2014 I saw a magazine advertisement for the latest arrival from Sky-Watcher: the Star Adventurer.
What struck me immediately about this product is its “small mount” character, which allows it to be used — with the appropriate accessories — not only with cameras but also with small telescopes.
After some research on the internet I decided to buy the Full Optional package: star tracker, equatorial mounting head, double bar with declination movement, 1 kg counterweight.
First impressions
At first contact this instrument seems well made; the controls are minimal but clear: a power switch/speed selector, a Northern/Southern hemisphere selector and a pair of buttons for fast RA motion (12X), extremely useful when using the Star Adventurer as a mini mount.
Besides these controls there are several connectors: the mini USB input that allows the Star Adventurer to be connected to a PC for firmware updates or — very usefully — to be powered from a USB socket, the connector for the optional cable that controls the shutter of the most common DSLRs, and finally the ST4 autoguider input that allows RA corrections to be made.
The locking collars for the RA and DEC clutches (if you have the bar with declination movement) are well made and easy to use even while wearing gloves.
The accessory that provides the declination movement also has a fine adjustment knob, excellent for centring the field to be framed precisely even with the clutch locked.
On arrival the polar scope was well aligned with the axis of the mount, but the graduated scale for positioning the polar reference was completely off; even so, about ten minutes were enough — following the instructions in the manual (unfortunately, for now, only in English) — to carry out the calibration.
It is important to perform this test before starting to photograph, because tracking accuracy depends heavily on how accurately the celestial pole is targeted: inaccurate alignment translates into a strong drift in declination, visible at longer focal lengths even after just a few minutes.
The first test
Being impatient, I decided to test the Star Adventurer straight away in mini-mount mode with a Coronado PST.
Assembly proved very simple and immediate: the equatorial head, with a 1/4" photographic thread, has a Vixen dovetail with its bar, which is fixed to the thread of the star tracker using the Allen key included in the box.
At this point you attach the bar with the declination support, screw on the counterweight kit, secure the telescope via the 3/8" photographic attachment, balance it, and the job is done.
Just insert 4 AA batteries and the instrument is ready for use: aligning the pole roughly with a compass and setting the Star Adventurer to solar tracking rate, it was possible to follow our star for an hour, occasionally making simple corrections in declination (made easier by the fine adjustment knob) caused by the approximate polar alignment.
In the field
It is in the field, however, that a star tracker must prove its worth, following the starry vault with precision.
Polar alignment is greatly simplified by the polar scope built into the RA axis: it is the same model found in the bigger sisters of the Sky-Watcher family, with a graduated scale that, once calibrated, allows the position of Polaris to be found simply by setting the date, the time and the difference in longitude with respect to the central reference meridian.
The only problem is that in order to fit the indispensable reticle illuminator you have to remove the ball head or the latitude bar.
For this reason it is advisable to mount the Star Adventurer on a solid tripod capable of not flexing when the camera and any counterweight are fitted.
The fine adjustments in azimuth and altitude allow Polaris to be positioned precisely on the reticle; the azimuth adjustment is very precise, while the altitude one shows a little play which, however, is not too annoying and, once the clutches are tightened, does not affect pointing accuracy.
Once polar alignment is done, you simply mount the camera, point at the subject and start tracking to begin photographing.
The switch, reminiscent of the rotary dial of old telephones, allows different rates to be set: sidereal, lunar, solar and 0.5X make the classic astronomical photographs possible, while the 2x, 6x and 12x rates allow the instrument to be used as a rotating dolly for time-lapse photography.
When a rate is selected its indicator glows faintly red, so that even in the dark it is really difficult to get it wrong.
Before presenting the results I obtained, I want to stress one thing: my considerations are the result of a few tests on a single unit and therefore must not be regarded as statistically accurate, nor as representative of the entire production run.
To put the instrument to the test I wanted to “start tough”: the first evening with a Sigma 28-300 zoom set to 80 mm, the second — more demanding — with a 180 mm Elmarit telephoto lens, for a total weight (camera + optics) of 1.9 kg.
The Star Adventurer was powered by a small external power tank, to which the camera was also connected.
The camera used was a Canon 350D with 6.4 µm pixels and, in both cases, the exposure time was 3 minutes; with both configurations, none of the individual exposures turned out to be trailed.
The following table summarises the conditions of two of the tests carried out: the best and the worst.
| Test | Focal length | Declination of subject | Total time | Image scale [arcsec/pixel] | Overall trailing | Trailing in RA | Max exposure* |
|---|---|---|---|---|---|---|---|
| Best | 180 mm | 58°N | 2.7 hours | 7.3 | 48 px / 5.84 arcmin | 26 px / 3.16 arcmin | 6.8 min |
| Worst | 80 mm | 43°N | 2 hours | 16.6 | 44 px / 12.2 arcmin | 40 px / 11 arcmin | 5.5 min |
* Maximum exposure calculated allowing a maximum trailing of 2 pixels
 |  |
|---|---|
| Trace from the 80 mm test | Trace from the 180 mm test |
Since the two tests were conducted on different days, repositioning the SA between one test and the other, it is difficult to make a direct comparison between them and hard to understand what caused the large difference between the two.
The only significant difference is that the battery used to power the star tracker externally during the test that gave the worse result had a lower charge level — so much so that, at the end of the test, it gave the low-battery signal: this may have affected tracking accuracy (and perhaps caused the curved trace).
The battery was then recharged before the better test.
Taking the better test as a reference, therefore, the following table of maximum exposure times can easily be extrapolated.
Maximum exposure times in minutes, accepting 2 pixels of trailing
| Focal length [mm] \ Declination [°] | 0 | 20 | 40 | 60 | 80 |
|---|---|---|---|---|---|
| 18 | 36 | 38.3 | 47 | 72.1 | 207.5 |
| 28 | 23.2 | 24.7 | 30.2 | 46.3 | 133.4 |
| 50 | 13 | 13.8 | 16.9 | 25.9 | 74.7 |
| 80 | 8.1 | 8.6 | 10.6 | 16.2 | 46.7 |
| 135 | 4.8 | 5.1 | 6.3 | 9.6 | 27.7 |
| 180 | 3.6 | 3.8 | 4.7 | 7.2 | 20.8 |
| 200 | 3.2 | 3.5 | 4.2 | 6.5 | 18.7 |
It is also easy to notice that during the better test the trailing occurred mainly in declination, because of the imprecision of the polar alignment.
It is therefore necessary to carry out the most accurate polar alignment possible while at the same time using a sturdy tripod capable of supporting the equipment without flexing.
Here are some shots taken with this useful instrument, available at full resolution on AstroBin:
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
- Image on AstroBin
The functions I did not test
During the test I was not able to try all the functions of this versatile object.
First of all the autoguiding function which allows, through the ST4 port, a second telescope to be connected to make RA corrections, lengthening the minimum exposure times, especially at longer focal lengths.
Nor did I try the function that allows a shutter-release cable to be connected to the camera: given the way it is designed, however, in my opinion this function has limited usefulness.
The preset intervals and exposure times (based on the tracking rate) cannot in fact be customised.
Conclusions
During these very first tests the Star Adventurer proved to be an excellent instrument.
Easy to assemble and to operate, it makes it possible to take spectacular deep-sky images even with decidedly long exposure times if you shoot at short focal lengths.
The ability to mount small telescopes makes it a versatile instrument even for those who are not involved in astrophotography, and turns it, in effect, into a mini mount.
Its compactness means it can be taken practically anywhere: it was possible to fit it, with all its accessories, into an aluminium case smaller than a briefcase.
If it could speak it would say “Don’t call me a star tracker!”.
In Italy the Star Adventurer is distributed by the company Auriga.